U.S. patent number 4,680,499 [Application Number 06/849,833] was granted by the patent office on 1987-07-14 for piezoelectric ultrasonic transducer with acoustic matching plate.
This patent grant is currently assigned to Hitachi, Ltd., Hitachi Medical Corporation. Invention is credited to Kageyoshi Katakura, Chitose Nakaya, Hiroshi Takeuchi, Shin'ichiro Umemura.
United States Patent |
4,680,499 |
Umemura , et al. |
July 14, 1987 |
Piezoelectric ultrasonic transducer with acoustic matching
plate
Abstract
A monolithic array ultrasonic transducer has a plurality of
transducer elements formed thereon by isolating metallized areas on
a piezoelectric plate without cutting the piezoelectric plate apart
for each transducer element, and an acoustic matching layer having
a longitudinal wave velocity within .+-.25% of a longitudinal wave
velocity of the piezoelectric plate and a thickness equal to one
half of that of the piezoelectric plate. The acoustic matching
layer suppresses the radiation to an object of a partial wave in a
direction of 60.degree. to a normal line to the plane of the
piezoelectric plate.
Inventors: |
Umemura; Shin'ichiro (Hachioji,
JP), Takeuchi; Hiroshi (Matsudo, JP),
Katakura; Kageyoshi (Tokyo, JP), Nakaya; Chitose
(Tokyo, JP) |
Assignee: |
Hitachi, Ltd. (Tokyo,
JP)
Hitachi Medical Corporation (Tokyo, JP)
|
Family
ID: |
26415435 |
Appl.
No.: |
06/849,833 |
Filed: |
April 9, 1986 |
Foreign Application Priority Data
|
|
|
|
|
Apr 10, 1985 [JP] |
|
|
60-74289 |
Aug 30, 1985 [JP] |
|
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60-189662 |
|
Current U.S.
Class: |
310/334; 310/327;
310/366; 73/644 |
Current CPC
Class: |
G10K
11/02 (20130101) |
Current International
Class: |
G10K
11/00 (20060101); G10K 11/02 (20060101); H01L
041/08 () |
Field of
Search: |
;310/322,366,334-337,800,327 ;73/642,644 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Budd; Mark O.
Attorney, Agent or Firm: Antonelli, Terry & Wands
Claims
We claim:
1. An ultrasonic transducer comprising:
a piezoelectric plate having both surfaces thereof metallized and
at least one of the surfaces having a plurality of isolated
metallized areas;
a backing material formed on the back surface of said piezoelectric
plate; and
an acoustic matching plate formed on the front surface of said
piezoelectric plate, having a longitudinal wave velocity within
.+-.25% of a longitudinal wave velocity of said piezoelectric plate
and having a thickness equal to one half of a thickness of said
piezoelectric plate.
2. An ultrasonic transducer according to claim 1, wherein the
longitudinal wave velocity of said acoustic matching plate is
within .+-.15% of the longitudinal wave velocity of said
piezoelectric plate.
3. An ultrasonic transducer according to claim 1, wherein said
acoustic matching layer is made of polymethylole melamine
resin.
4. An ultrasonic transducer comprising:
a piezoelectric plate having both surfaces thereof metallized and
at least one of the surfaces having a plurality of isolated
metallized areas;
a backing material formed on the back surface of said piezoelectric
plate;
a first acoustic matching layer formed on the front surface of said
piezoelectric plate and having a longitudinal wave velocity within
.+-.25% of a longitudinal wave velocity of said piezoelectric plate
and a thickness equal to one half of a thickness of said
piezoelectric plate; and
a second acoustic matching layer formed between said piezoelectric
plate and said backing material and having a longitudinal wave
velocity within .+-.25% of the longitudinal wave velocity of said
piezoelectric plate and a thickness no larger than 1/4 of a
thickness of said piezoelectric plate.
5. An ultrasonic transducer according to claim 4, wherein the
longitudinal wave velocities of said first and second acoustic
matching layers are within .+-.15% of the longitudinal wave
velocity of said piezoelectric plate.
Description
BACKGROUND OF THE INVENTION
The present invention relates to an ultrasonic transducer suitable
for a sensor in an ultrasonic imaging device such as ultrasonic
diagnostic device or ultrasonic deflect detectors.
An array type ultrasonic transducer having a monolithic
piezoelectric plate (monolithic array transducer) inherently has a
high performance and a low manufacturing cost which are compatible.
One example thereof is shown in U.S. Patent Application Ser. No.
676,314 filed in 1984 by the inventors of the present invention. In
this type of transducer, since a transducer element of the array is
not mechanically cut, a partial wave which laterally propagates
along the piezoelectric plate is generated, which degrades an image
quality.
SUMMARY OF THE INVENTION
It is an object of the present invention to provide an ultrasonic
transducer which resolves the problem inherent to the monolithic
array transducer and can provide a high quality of image with a low
cost.
In order to achieve the above object, the monolithic array
transducer of the present invention comprises a monolithic
piezoelectric plate and an acoustic matching layer formed on a
surface of the piezoelectric plate and having approximately one
half of a thickness of the piezoelectric plate and made of a
material having a substantially equal longitudinal wave velocity to
that of the piezoelectric plate.
The material of the acoustic matching layer is selected such that
it has a longitudinal wave velocity which is within .+-.25% of that
of the piezoelectric plate. Preferably, it is within .+-.15%.
In accordance with the arrangement of the present invention,
partial waves generated in the piezoelectric plate in directions
other than normal to the plane of the piezoelectric plate are
suppressed from being radiated to an object so that the transducer
can provide a high quality of image. More specifically, an acoustic
wave which is normal to the plane of the acoustic piezoelectric
plate which has a thickness equal to .lambda./2, where .lambda. is
a wavelength of the acoustic wave used, as well as partial waves in
various directions are generated in the piezoelectric plate. Of
those partial waves, the partial wave in a direction in which an
acoustic path length in the piezoelectric plate is .lambda., that
is, in a direction of 60.degree. to a normal line to the plane of
the piezoelectric plate is strongest. In the prior art, the
acoustic matching layer has a thickness of .lambda./4 and is
designed to radiate the acoustic wave normal to the plane of the
piezoelectric plate most efficiently. In the prior art acoustic
matching layer, since the longitudinal wave velocity is lower than
that of the piezoelectric plate, the partial wave in the direction
of 60.degree. propagates at a smaller angle in the acoustic
matching layer. Accordingly, such partial wave is radiated to the
object with a fairly high efficiency. On the other hand, in
accordance with the present invention, the partial wave in the
direction of 60.degree. propagates in the direction of
substantially 60.degree. in the acoustic matching layer. Therefore,
the acoustic matching layer has a path length substantially equal
to .lambda./2 to the partial wave. As a result, the partial wave is
essentially not radiated to the object.
Thus, in accordance with the present invention, the radiation of
the strongest partial wave to the object is suppressed and the
transducer can attain a high quality of image.
BRIEF DESCRIPTION OF THE DRAWINGS
FIGS. 1 and 2 show perspective view and sectional view of one
embodiment of the present invention, and
FIGS. 3 and 4 show perspective view and sectional view of another
embodiment of the present invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
In an embodiment shown in FIG. 1, an acoustic matching layer 2
having a thickness approximately one half of a thickness of a
piezoelectric plate 1 is formed on a front surface of the
piezoelectric plate 1, and a backing material 3 is formed on a back
surface of the piezoelectric plate 1. One surface of the
piezoelectric plate 1 is metallized to have stripes 11, and the
other surface is metallized over the entire surface. In this
manner, a monolithic array transducer having a plurality of
transducer elements arranged on one piezoelectric plate is
provided.
The present embodiment is intended to transmit and receive an
acoustic wave to and from a living body (acoustic impedance
1.5.times.10.sup.6 kg/m.sup.2.sec), and a PZT ceramic
(lead-zirconate-titanate) having a longitudinal wave velocity of
3800 m/sec, an acoustic impedance of 28.times.10.sup.6
kg/m.sup.2.sec and a thickness of 0.7 mm is used as the
piezoelectric plate a resonance frequency of the transducer is 2.7
MHz. On the other hand, a poly-methylole melamine resin having a
thickness of approximately 0.35 mm, a longitudinal wave velocity of
3300 m/sec and an acoustic impedance of 5.times.10.sup.6
kg/m.sup.2.sec is used as the acoustic matching layer 2 which is
formed on the piezoelectric plate 1. The above-mentioned melamine
resin may exemplarily have a molecular formula such as ##STR1##
Rubber having powders of metal oxide mixed thereto is used as the
backing material 3.
By using the acoustic matching layer 2 having the essentially equal
longitudinal wave velocity to that of the piezoelectric plate 1,
the radiation of the partial wave to the object, which would be
radiated obliquely from the piezoelectric plate, is suppressed.
This will be explained with reference to FIG. 2. The thickness T of
the piezoelectric plate 1 is given by
where f.sub.r is a resonance frequency, .lambda. is a wavelength
and C is a longitudinal wave velocity. When the piezoelectric plate
is excited at the frequency f.sub.r, an acoustic wave normal to the
plane of the piezoelectric plate as well as partial waves in
directions of .theta. to the normal line are generated. Of those
partial waves, the partial wave in the direction of T/cos
.theta.=.lambda. or .theta.=60.degree. is strongest. This partial
wave 21 is repeatedly reflected by the front surface and the back
surface of the piezoelectric plate 1 and propagates laterally. When
the acoustic velocities of the piezoelectric plate 1 and the
acoustic matching layer 2 are substantially equal, a portion of the
partial wave is not essentially refracted at the interface and goes
into the acoustic matching layer 2. Since the acoustic matching
layer has a thickness of .lambda./4, a path length of the partial
wave in the acoustic matching layer is .lambda./4.multidot.1/cos
.theta.=.lambda./2. Accordingly, this partial wave is not
essentially radiated from the acoustic matching layer 2 to the
object.
On the other hand, the prior art .lambda./4 acoustic matching layer
has a much lower longitudinal wave velocity than that of the
piezoelectric plate. Accordingly, the partial wave in the direction
of 60.degree. propagates at a smaller angle in the acoustic
matching layer by refraction. Thus, the path length is shorter than
.lambda./2 and the partial wave is radiated to the object with a
high efficiency.
In order to effectively suppress the emission of the partial wave
to the object, it is necessary that the longitudinal wave velocity
of the acoustic matching layer is within .+-.25% of that of the
piezoelectric plate. The effect is remarkable if it is within
.+-.15%. When the lead-zirconate-titanate (PZT ceramic) (having
longitudinal wave velocity of 3800 ms) is used as the piezoelectric
plate, the materials of the acoustic matching layer which meets the
above requirement are polymethylole melamine resin and glass (trade
name EDF-4, longitudinal wave velocity 3700 m/sec). When a lead
titanate (PbTiO.sub.3) ceramic (longitudinal wave velocity 4400
m/sec) is used as the piezoelectric plate, the polymethylole
melamine resin or the glass described above may also be used as the
acoustic matching layer. The above glass has an acoustic impedance
of 17.4.times.10.sup.6 kg/m.sup.2.sec which is too high to the
impedance matching between the piezoelectric ceramic and the living
body. An excellent result is obtained by laminating the resin
acoustic matching layer on the glass acoustic matching layer.
In any case, it is most desirable that the thickness of the
acoustic matching layer is .lambda./4 if the propagation efficiency
of only the wave normal to the plane is considered. However, from
the standpoint of the suppression of the partial wave radiation, it
is desirable that the thickness of the acoustic matching layer is
not exactly .lambda./4 but is T/2 irrespective of a difference
between the velocities, where T is a thickness of the piezoelectric
plate.
The polymethylole melamine resin used in the above embodiment is
easy to be formed and has a high acoustic velocity among the
polymer materials. As a result, the acoustic impedance is as high
as 5.times.10.sup.6 kg/m.sup.2.sec and it can be used as the
acoustic matching layer, without anything mixed, between an
electroacoustic transducer material such as piezoelectric ceramics
and a medium such as water or human body. Accordingly, the acoustic
matching layer can advantageously be obtained having a higher
uniformity than the prior art acoustic matching layer made of epoxy
resin having metal particles or metal oxide particles mixed
therewith to increase its specific gravity.
FIG. 3 shows another embodiment of the present invention. The
present embodiment differs from the embodiment of FIG. 1 in that a
second acoustic matching layer 4 having a thickness of T/4 is
formed between the piezoelectric plate 1 and the backing material
3. The structures and materials of other portions are identical to
those of the embodiment of FIG. 1. The second acoustic matching
layer 4 is made of glass (trade name EDF-4, longitudinal wave
velocity 3700 m/sec).
FIG. 4 illustrates the function of the second acoustic matching
layer 4. The partial wave in the direction of 60.degree. is
reflected on the surface of the piezoelectric plate 1 and radiated
to the backing material 3 from the back surface through the second
acoustic matching layer. Since the path length of the partial wave
in the acoustic matching layer 4 is substantially .lambda./4, the
partial wave 22 is efficiently directed to the backing material 3
and is absorbed thereby. As a result, the affect by the partial
wave is further suppressed than in the first embodiment. The
longitudinal wave velocity of the acoustic matching layer 4 to
attain the above effect is within .+-.25% of the longitudinal wave
velocity of the piezoelectric plate 1, and more preferably within
.+-.15%.
* * * * *